Most high-quality contemporary audio instruments have the input configuration of figure 6. When the two input terminals are floating, the differential amplifier provides common-mode rejection. This can effectively remove the problems of hum and ground loops. The common-mode input capacitance (both inputs connected together) to ground was measured to be 130pF for the Sound Technology 1700B and 700pF for the A-P System 2.

Note: to avoid winding inductance affecting the capacitance measurement, the primary should be shorted while measuring - this kills the inductance without affecting the capacitance.

    Older or less-expensive equipment have the input configuration of figure 7. No common-mode rejection is provided, so it is usually necessary to break any ground loops at the oscillator end by using oscillators such as the H-P 200CD.

Test Discrepancies
     Several test runs were made on several output transformers with different pieces of test equipment and different grounding arrangements. It quickly became clear that when the transformer secondary was floating, the high frequency response was greatly extended and even peaked. However, there was much variation from one piece of equipment to another. Some examples just using the System 2 in different configurations make the differences clear. Note that all tests were run with Rs = 1.5K and RL = 8 ohms.
    Figure 8 shows high frequency sweeps with connections as shown. The transformer is an experimental 3K single-ended transformer with excellent low frequency response (not shown) and extensive interleaving. The measured primary-to-secondary capacitance is 0.012uF.